Question

CaH2(s) + 2 H2O(l)--->Ca(OH) 2(s) + 2 H2(g)
After the reaction above, 90.0 mL of H2(g) was collected over water at 25 °C and a total pressure of 755.0 mm Hg. How many grams of CaH2 decomposed?
The vapor pressure of H2O at 25 °C is 23.8 mm Hg?

Answer 1

@sweetburger

Answer 2

P:.9621atm
V:.09L
n:m/M
R:.0821 L x atm/K x mol
T:298.15K
ok so I have to find grams of NaH2 so I made n=mass/molar mass
m=MPV/RT
and got .08847....g NaH2 this feel and looks wrong..

Answer 3

@Photon336

Answer 4

NaH2:25.01g

Answer 5

using what numbers?

Answer 6

RT/PV=
how did you get .9934atm and 9x10^-2?

Answer 7

we have 90 ml right?

Answer 8

90.0ml x 1L/1000mL=.09L

Answer 9

yep. and I just wrote that in scientific notation.

\[9.0*10^{-2} L \]

Answer 10

but i guess it doesnt matter how you write it

Answer 11

how about .9934?

Answer 12

what about the partial pressure? 755-23.8?

Answer 13

that's interesting. I'm thinking that we would need the total pressure for this

Answer 14

what did you have in mind?

Answer 15

n=PV/RT
.9621x.09/298.15x.0821=.003537 mols of H2

Answer 16

right?
.003537 H2 x 1molCaH2/2molsH2 x 25.01gCaH2/1molCaH2

Answer 17

I used stoichiometry to find grams of CaH2

Answer 18

I got .044...

Answer 19

I give up >.<

Answer 20

let's start with the formula again

PV = nRT

\[ \frac{ PV }{ RT } = n \]

Answer 21

\[\frac{ \frac{ 731.2~mmHG }{ 760 } *(\frac{ 90 }{ 1000 }L )}{ 0.08*298K } = moles~H_{2}\]

Answer 22

\[3.63*10^{-3}~mol~H_{2}*(\frac{ CaH_{2} }{ 2H_{2}})*(\frac{ 42.094~Grams }{ Mole~CaH_{2} }) = 7.64*10^{-2}~grams \]

0.076 grams

Answer 23

wait wheres ur R?

Answer 24

you put .08 not .0821

Answer 25

omg I made a mistake

Answer 26

I just rounded it up to 0.08

Answer 27

I put 25.01g rather than 42.1g

Answer 28

yeah your answer is roughly half of mine

Answer 29

so 0.076 is correct?

Answer 30

yeah, this is under the assumption that we subtracted the pressure of water from the total pressure.

Answer 31

ya I kept thinking it was NaH2 but its CaH2 ;P

Answer 32

thanks man your the best

Answer 33

yeah no problem. you knew what you were doing